Actuators and magnetism

There are essentially two types of mechanical movement actuators: one type is the solenoid and the second is the electric motor. There are a number of variations in solenoids and electric motors, but, in general, solenoids are used to achieve linear movement and motors are used for rotary movement (although it is possible for motors to be used to create linear movement, via a mechanical mechanism, or it is possible for solenoids to create rotary movement, via a linkage).

The operation of mechanical actuators (solenoid and electric motor types) relies on magnetism. Hillier’s Fundamentals of Motor Vehicle Technology Book 3 explains in detail the way in which magnetic fields are created and used for electric motors, solenoids and generators, etc. However, the essential fact is that, when a current is passed through a coil of wire, a magnetic field is created around that coil of wire. The magnetic field can then be used to create movement.

Solenoid type actuators

In a simple solenoid (Figure 1.40a), a soft iron plunger is located within the coil, but the plunger is free to move with a linear motion. When an electric current is passed through the coil of wire and the magnetic field is created, this will cause the plunger to be attracted towards or through the coil. When the current is switched off, the spring will return the plunger back to the start or rest position. Different designs and

Figure 1.39 Communication between engine management and automatic gearbox ECUs

Figure 1.40 Simple solenoids a Simple solenoid

b Double acting solenoid

constructions of solenoids allow many different tasks to be performed. For example, the double acting solenoid (Figure 1.40b) uses two coils of wire. One coil creates a magnetic field, which moves the plunger in one direction, and the other coil creates a magnetic field, which moves the plunger in the opposite direction.

It is also possible for the ECU to regulate or control the average current flow and voltage passing through the coil of wire by altering the duty cycle and frequency of the control signal pulses (see section 1.8). With this control process, it is possible to control or regulate the strength of the magnetic field. If the plunger is moving against a physical resistance such as a spring, it can be moved further by increasing the strength of the magnetic field. Reducing the magnetic field will result in the plunger moving back slightly. Additionally, when a double acting solenoid is used, the plunger movement can be controlled in both directions; in fact one magnetic field can be used to oppose the other. This allows an ECU to move and position the plunger with reasonable accuracy.

Solenoid plungers can be connected to a number of different types of mechanisms or devices that will perform different tasks or functions; various solenoid actuators are covered in the relevant chapters within this book.

Electric motor type actuators

A simple electric motor operates on similar principles to the solenoid, but instead of the magnetic field causing a plunger to move with a linear motion, the magnetic field forces a shaft to rotate. Figure 1.41 shows a simple electric motor, which in this example has a permanent horseshoe shaped magnet with a north and south pole.

A single loop of wire, which would normally be attached to a rotor shaft, is fed with an electric current, thus creating an electromagnetic field around the loop

Figure 1.41 Simple electric motor. Note that the primary and secondary windings are wound around a soft iron core to concentrate and intensify the magnetic field

a Current passes from A to B creating north and south poles on the electromagnet. The like poles will cause the shaft and wire loop to rotate

b When the rotor has turned through 180°, the commutator arrangement causes the current to flow in the reverse direction around the wire loop (from B to A), therefore changing the north pole to a south pole and the south pole to a north pole. The like poles will again repel and cause the shaft to rotate through another 180°

of wire. When the electromagnetic field is created, north and south poles will exist around the loop of wire.

These north and south poles will either be attracted to or repelled from the north and south poles of the permanent magnet. Remember that like poles repel each other and unlike poles attract each other.

When the current is initially passed through the wire loop, e.g. from connection A to connection B on the wire loop, if the electromagnet north pole is adjacent to the permanent magnet north pole (and the two south poles will also be adjacent to each other), this will force the shaft to rotate (Figure 1.41a). When the shaft then rotates through 180º, the north poles will be adjacent to the south poles, and because unlike poles attract each other, the motor will not rotate any further.

However, in the diagram it can be seen that the pair of semi-circular segments (or commutator) is attached to the ends of the wire loop and therefore rotates with the loop. The electric current passes from the power supply to contact brushes which rub against the segments as the shaft rotates. Therefore, when the shaft and the segments have rotated through 180º, the two segments are now not in contact with the original brushes, but they are in contact with the opposing brushes. This means that the electric current will be flowing from connection B to connection A (Figure 1.41b), which is in the opposite direction around the wire loop. The result is that the north pole of the electromagnet is now a south pole, and the south pole is now a north pole, which will cause the shaft and wire loop to rotate another 180º; the process is then repeated.

The simple electric motor in Figure 1.41 shows how magnetism can provide continuous rotary movement;

the resulting rotary motion can operate various devices.

Simple examples include fuel or air pumps, and wiper motors operate on the same principles.

However, many of the electric motors used on ECU controlled vehicle systems are often more complex and sophisticated in the tasks they have to perform, and in their design and construction. Many of the motors do not in fact perform a complete rotation, or they may be controlled so that they rotate in small angular steps.

These types of motors are controlled by using different types of wire loops (usually coils of wire) and using different designs of commutator. In addition, by applying control signals from the ECU that have changing duty cycles, pulse widths and frequencies, it is possible to rotate motors partially so that they start and stop in any desired position. The partial rotation can be progressive from one position to another, or it can be achieved in a series of steps.

The capacity to control the rotation of motors accurately allows them to be used for a variety of tasks such as opening and closing air valves in small increments (used for idle speed control). Other examples of ECU controlled motors are dealt with individually in the following sections and in other chapters of this book.

Magnetism and non-mechanical actuators

There is one main actuator used on motor vehicles that uses the effects of a magnetic field but does not produce mechanical movement – this is the ignition coil.

An explanation of how an ignition coil works is provided in Chapter 2 of Hillier’s Fundamentals of Motor Vehicle Technology Book 1. It is sufficient here to highlight the basic principles of ignition coil operation, which rely on the movement of a magnetic field or magnetic flux to induce an electric current into a coil of wire.

When a current is passed thorough a coil of wire, it creates a magnetic field; this is the same principle as used in electric motors. Additionally, as is the case with an electrical generator, when a magnetic field moves through a coil of wire (or the coil is passed through a magnetic field) it causes an electric current/voltage to be generated within the coil of wire. The faster the magnetic field moves relative to the wire, the greater the voltage produced. An ignition coil relies on both processes.

On most vehicles, the voltage in the vehicle electrical system is only around 12 volts, which is not sufficient to create a spark or electric arc at the spark plug gap. The ignition coil must provide a way to increase the voltage from 12 volts to many thousands of volts. A principle that is used in electrical transformers is also used for ignition coils: there are two coils of wire, one of which has many more windings than the other.

In an ignition coil a secondary coil can typically have 100 times more windings than the primary coil (see Figure 1.42).

The process

The process relies on current (using the vehicle’s 12 volt supply) passing through the smaller coil or primary winding to create a magnetic field. The build up of the magnetic field is relatively slow, but once the magnetic

field has been established at full strength, it can be maintained for a very brief period so long as the current continues to flow. However, when the current is switched off, the magnetic field collapses extremely rapidly, in fact very much more quickly than the speed at which it was created.

Whilst the magnetic field is collapsing, the lines of magnetic force are collapsing across the same coil of wire that created it (primary winding); this causes a current/voltage to be produced within the primary winding. Because the speed of collapse of the magnetic field is very rapid, it causes a much higher voltage to be produced within this coil of wire, sometimes as high as 200–300 volts. Therefore, the speed of collapse is used to step up the voltage from 12 to typically 200 volts.

However, 200 volts are still not sufficient to provide the spark at the spark plug under the conditions that exist in the combustion chamber (high pressure and other factors make it difficult for an arc to be created at the plug gap).

To achieve the desired voltage necessary to create the spark, a secondary winding is used, as mentioned above. The secondary winding can be adjacent to the primary winding, although one winding is often wrapped around the other. When the magnetic field is created, the secondary winding is also exposed to the magnetic field. Therefore, when the magnetic field collapses, as well as creating a voltage in the primary winding, it also creates a voltage in the secondary winding. Because the secondary winding may have 100 times the number of turns or windings, 100 times the voltage can in theory be produced. If 200 volts could be produced in the primary winding (owing to the rapid speed of collapse of the magnetic field), then in the secondary winding it should theoretically be possible to produce 20 000 volts (100 times greater).

Figure 1.42 Simple construction of an ignition coil

For most petrol engines, the required voltage to produce a spark at the spark plug (under operating conditions) is around 7 000 to 10 000 volts; therefore a coil that is able to produce 20 000 volts is more than capable of producing a spark. There is therefore sufficient additional voltage available to overcome many minor faults such as a plug gap that is too large or contaminated.

Actuators convert electrical signals into actions Common actuators, such as fuel injectors, are solenoid operated

Key Points